Archive for August, 2008

Sustained IO on EBS == No Bueno

Wednesday, August 27th, 2008

I have a small EC2 instance running with a 25GB EBS volume attached. It has a database on it that I need to manipulate by doing things like dropping indexes and creating new ones. This is on rather large (multi-GB, millions of rows) tables. After running one DROP INDEX operation that ran all day without finishing, I killed it and tried to see what was going on. Here’s the results of the first 10 minutes of testing:

-bash-3.2# dd if=/dev/zero of=/vol/128.txt bs=128k count=1000
1000+0 records in
1000+0 records out
131072000 bytes (131 MB) copied, 0.818328 seconds, 160 MB/s

This looks great. I’d love to get 160MB/s all the time. But wait! There’s more!

-bash-3.2# dd if=/dev/zero of=/vol/128.txt bs=128k count=100000
dd: writing `/vol/128.txt': No space left on device
86729+0 records in
86728+0 records out
11367641088 bytes (11 GB) copied, 268.191 seconds, 42.4 MB/s

Ok, well… that’s completely miserable. Let’s try something in between.

-bash-3.2# dd if=/dev/zero of=/vol/128.txt bs=128k count=10000 
10000+0 records in
10000+0 records out
1310720000 bytes (1.3 GB) copied, 15.4684 seconds, 84.7 MB/s

So the performance gets cut in half when the number of 128k blocks is increased 10x. This kinda sucks. I’ll keep plugging along, but if anyone has hints or clues, let me know. If this is the way it’s going to be, then this is no place to run a production, IO-intensive (100,000s and maybe millions of writes per day, on top of reads) database.

More Adventures in Amazon EC2 (and EBS)

Wednesday, August 27th, 2008

Short Version: You can find a fantastic video here about bundling customized AMIs and registering them with Amazon so that you can launch as many instances of your new AMI as you want. The video is so good that I don’t bother writing out the steps to do the bundling (it would be pretty darn long). These are some notes about launching an AMI, customizing it, and mounting an EBS volume to it (the video linked above doesn’t cover EBS). Also, check out the ElasticFox tool which is a very good GUI for doing simple EC2 operations. Nice if you’re just getting started or doing some simple tests.

There are two ways you can go about creating a custom machine image (AMI) for use with Amazon EC2: You can create an image locally by dd’ing to a file, mounting it with “-o loop” creating a filesystem on it, and bootstrapping the whole thing yourself, or you can grab an existing AMI that will serve as a “good enough” base for you to make your customizations, then bundle the customized image.

I’ll be talking about the latter option, where you identify a “good enough” image, customize it for your needs, and save that as your AMI. Unless you’re doing some kind of highly specialized installation, or are a control freak, you shouldn’t really need to start from scratch. I was just building a test image, and wanted a CentOS 5.2 base installation.

Here’s the command you can use to browse the AMIs you have access to (they’re either public, or they’re yours):

$ ec2dim -a

If that command looks funny to you, it’s likely because you’re used to seeing the really long versions of the AWS commands. Amazon also provides shorter versions of the commands. No, really – have a look! The long version of this command is:

$ ec2-describe-images -a

Too long for my taste, but it’s nice to know it’s there.

So, rather than start from scratch, I grabbed a base image that was close enough for my needs, and customized it. It’s a 5.1 base image, pretty well stripped of things that I don’t need, and a few that I do, but that’s ok. I’d rather start with less than more.

So step one is to launch an instance of the AMI I’ve chosen to be my ‘base’. Simple enough to do:

$ ec2run ami-0459bc6d -k ec2-keypair

And that’s pretty much it. It takes a couple of minutes (literally) for the machine to actually become available. You can check to see if it’s still in “pending” state or if it’s available by running ‘ec2din’. Without arguments, that’ll show you the status of any instances you have pending or runnning. Once the instance is running, you’ll be able to glean the hostname from the information provided.

An important note at this point: Don’t confuse “image” with “instance”. For the OO types in the crowd, an “image” is an object. It does nothing by itself until you instantiate it and create an “instance” of that object. For sysadmins, the “image” is like a PXE boot image, which does nothing until you boot it, thereby creating an “instance”.

The reason I used “PXE” and “object” in the above is because of the implication it makes: you can launch as many instances of an object as you want from a single object definition. You can boot as many machines as you want from a single PXE boot image. Likewise, you can launch as many Amazon EC2 instances from an image as you want.

So, in the time it took you to read those last two paragraphs, your instance is probably running. I now grab the hostname for my instance, and ssh to it using my keypair:

$ ssh -i ec2-keypair root@<hostname>

Now that I’m in, I can customize the environment, and then “bundle” it, which will create a new AMI with all of my customizations. With the instance in question, I installed a LAMP stack, and a few other sundry tools I need to perform my testing. I also ran “yum -y upgrade” which will go off and upgrade the machine to CentOS 5.2.

One thing I want to do with this instance is test out the process for creating an EBS volume. The two pieces of information I need to do this are the size of the volume I want to create, and the “zone” I want to create it in. You can figure out which zone your instance is running in using ‘ec2din’ on your workstation (not in your instance). I took that information and created my image in the same zone using the ‘ec2addvol’ command. If you don’t have that command on your workstation, then you don’t have the latest version of the Amazon command line tools. Here’s the command I ran:

$ ec2addvol -z us-east-1b -s 25

To see how it went, run ‘ec2dvol’ by itself and it’ll show you the status of all of your volumes, as well as the unique name assigned to your volume, which you’ll need in order to attach the volume to your instance. To do the ‘attachment’, you need the name of the volume, the name of the instance (use ‘ec2din’), and you need to choose a device that you’ll tell your instance to mount. Here’s what I ran (on my workstation):

$ ec2attvol -d /dev/sdx -i i-xxxxxxxx -v vol-xxxxxxxx

Now you can go back to the shell on your instance, mount the device, create a file system, create a mount point, add it to fstab, and, as they say in the UK, “Bob’s yer uncle”. By the time I wrote this post, I had already shut down my instance, but here are the commands (caveat emptor: this is from memory):

# mkfs.ext3 /dev/sdx
# mkdir /vol
# mount /dev/sdx /vol

If that all works ok, you can add a line to /etc/fstab so that it’ll be mounted at boot time, but I haven’t yet figured out how to attach a volume to an instance at boot time. The mount doesn’t work if you don’t attach the volume to the instance first. You’ll get a “device doesn’t exist” error if you try it. Clues hereby solicited. I assume I could probably use ‘boto’ and some Python code to get this done, but doing the same with a shell script wrapper around the Amazon tools might also be possible — but I don’t know how reliable that would be, because you’re at the mercy of Amazon and how they decide their tools should present the data (and *if* they provide the data you need for a particular operation down the road).

So now I have an EBS volume, and an instance. The volume is attached to the instance, and I can do things with it. I’m testing some database stuff, so I copied a database over to the volume, which was now mounted, so I could just ‘scp mydb.tbz root@<instance>:/vol/.’

Once my database is there, I can attach it to pretty much whatever I want, which makes it nice, because I can test the same database, and the same database code, and see how the different size Amazon instances affect the performance, which gives me more performance data to work with. For production purposes, I’ll have to look more closely at the IO metrics, play with attaching multiple volumes and spreading out the IO, and I also want to test the ‘snapshot’ capabilities. It’s also nice to know that if I needed to launch this in production (there are no plans to do so, but you never know), I could upgrade the database “hardware” more or less instantly 😀

If anyone has code or tools to help automate the management of all of this stuff, please send links! If I come up with any myself, I’ll most likely post it here.

Now that I have a customized AMI with all of my packages installed and my config changes made, I need to bundle this so that I can boot as many instances of this particular configuration as I want. An important note about bundling this *particular* image is that you MUST run ‘depmod -a; modprobe loop’ before bundling, since this process basically abstracts the manual process of bundling an image, which involves mounting a file as a volume, which requires a loopback mount.

The video I used to do the bundling is here, and if you can live through the disgustingly bad burps and chirps in the (Flash version) audio, it’s an excellent tutorial for bundling custom AMIs. While the process *is* pretty straightforward, it involves a number of steps, and the video goes through all of them, and it worked perfectly the first time through.

PyWorks Conference Schedule Posted

Wednesday, August 27th, 2008

Hi all,

The schedule for PyWorks has been posted! I’m really excited about three things:

1) there are some really cool talks that I’m looking forward to attending. There are a couple of sysadmin-related talks, AppEngine, TurboGears, Django, and an area I’ve been especially slow to move into: testing (I know, shame on me). There’s lots more so be sure to check it out.

2) the conference scheduling process is over 😉

3) I get to meet a lot of people face-to-face that I’ve worked with in the past on Python Magazine developing articles, or interacted with on IRC, etc. One thing I like about conferences surrounding open source technologies is you get to thank people face-to-face for the sweat they poured into some of the tools I use regularly. Mark Ramm, Kevin Dangoor, Michael Foord, Brandon Rhodes, and a collection of Python Magazine authors will be speaking there, and other Python Magazine folks and generally familiar faces will be in attendance.


For those still unaware, PyWorks will be held in Atlanta, Nov. 12-14, 2008. It’s sponsored by MTA, the publisher of Python Magazine, as well as php|architect. In fact, the php|works conference will be held simultaneously with PyWorks, and attendees of one are free to access talks in the other at will. There will also be a “center track” that will cover some more generic topics of interest to developers without regard to the language in use. Check it out!

Why I Don’t Write Book Reviews

Friday, August 8th, 2008

I have a lot of interaction with publishing types. I write a lot, and I edit some, and I do tech reviews and stuff for some publishers, and I co-authored a book, and I’ve worked on two magazines, and a newspaper, and I’m generally fascinated by the technical book market and stuff like that. I’m also someone who is lucky enough that his job is also his hobby. I work in technology, and am always doing something technology related at home in my spare time. Needless to say, I read tons upon tons of technical books.

I almost never post book reviews, in spite of the fact that I read all of these books. Why? Well, to be honest, I couldn’t tell you. It just hasn’t occurred to me to write a book review. Could be because I don’t really value book reviews too much myself I guess. I mean, if there’s a really obvious consensus across a huge number of reviews, I might be swayed. But in general, I find that book reviews are too often the target of astroturfing campaigns.

If there’s a tech book you’d like a review of that deals with things I’m generally into, let me know and I’ll post a review, if I’ve read it (or want to read it). Here are subjects I’m likely to have read books about in the past couple of years:

  • Linux, UNIX, and administration thereof
  • Python (all levels — I just read pretty much whatever is out there)
  • web 2.0 APIs (mostly Google and Amazon)
  • Any book about any service that can be run in a *x environment (DNS, Apache, DHCP, Jabber, and most other things that open a port)
  • Anything related to generic SQL, database design, or (more specifically) mysql and postgresql.
  • HPC (cluster computing)
  • Generic programming, software, computer science, or high-level systems design books
  • Digital photography (I have a Canon Digital Rebel, if that helps — I do *not* use Photoshop)
  • PHP
  • Maybe some other stuff I’m forgetting

Design Patterns in System Administration

Sunday, August 3rd, 2008

Most readers of my blog know that I consult, in addition to usually having a day job. I started my career working for a consulting firm, and couldn’t let go of the endless fascinating problems that exist in the “technological landscape”, and in addition, the seemingly endless numbers of ways to solve them. I’ve learned more than tons about how people, and institutions, approach technical problems in system design, and maybe more importantly, how they think about the problems and solutions.

I’ve worked in huge enterprises (several Fortune 100 companies), academia (, to be more exact), government (, for example), and a few startups and small businesses. I also grew up around small business around the time that technology was starting to become affordable enough to creep into even small offices (I helped run wire for my father’s first modem-connected office network around 1988 or so, and my mother’s office — admittedly much larger — had a mainframe and a few terminals, from which ascii posters of JFK and MLK were printed and hung on my walls when I was as young as 7 or 8). Observing and working with people to solve technical problems continues to bring me a lot of joy, and present plenty of challenges.

Over the years, I’ve done a decent bit of what I’ll loosely call “programming”. 10 years ago I might not have qualified that, but working for 6 years in support of graduate computer science research has a way of humbling a guy (and, really, for most grad students, actually *doing* 6 years of graduate research is probably just as humbling, if not more). One thing I’ve tried to do is keep up with trends in how programs are deployed, how the teams of workers in what are considered separate problem domains interact to get the applications to be useful to people, how the systems are organized, and how programs are designed, and finally, how to program…. um… “better” (for some undefined but surely long-winded definition of that term). As I’m starting to witness something of a convergence of programming and systems work (at least in my neck of the woods), programming is something I’m spending even more time doing, and learning.

Design patterns, whether in the context of extreme programming, agile methodologies, or whatever the project management philosophy is, appear to be extremely useful, but I’ve wondered why there doesn’t appear to be any movement in the system administration community toward defining some patterns for solving problems in the realm of systems infrastructure architecture. A few years back I stumbled upon, which I think is an excellent general methodology for building infrastructures, but I think a fuller treatment of the topic could be had. Preferably one that addresses a broader set of problems prevalent in a wider variety of environments. For example, I found the tools and methodologies there to map perfectly in government and academic environments, and portions of that work can be mapped onto small business problems, but it leaves enterprise environments, and some larger government environments with some unanswered questions or unaddressed problems.

I don’t blame the folks at — on the contrary, I applaud their work! But why has it been so difficult to find solutions to problems those nice folks just didn’t have, or didn’t have to focus on in their part of the organization?

So much of what we do is tribal knowledge, or knowledge earned “the hard way” — in the trenches, at 4am, on a Sunday, uphill, both ways… etc., but while many of these stories sound similar enough to discern a pattern, and while horror stories at conferences are universally met with “me too”, and “you should’ve done x, y and z, and it wouldn’t have been an issue”, I have yet to see these patterns codified in any meaningful way in a single work, or perhaps, an organized volume of works (no, mailing lists do *not* constitute an organized volume of works).

If something as complex and diverse as programming can have patterns applied to it, I have to believe that the same could hold true for building systems. If there were such a work, it could potentially serve as a de facto “best practices” reference — one that could be referred to by both technicians and higher-level decision makers, define a common language that both could understand, and help overcome some of the inevitable “people issues” that sysadmins (and, indeed, managers) often blame for a lack of forward movement.

Does such a work exist? Is this in the works now? Though I try to keep my finger on the pulse of the publishing market, I have yet to see any real commitment to the idea that a large swath of problems in systems can be solved using variants of pre-defined patterns. It’s not that we’re not using them, of course, and it’s not that there aren’t large numbers of us who could probably recite them off the top of our heads, but if you’re one of those people, you’re a “senior” system administrator (or better), and if that’s the case, imagine what your career might’ve been like if you had such a reference, and also, let me know what the “you” with 1 year of sysadmin experience would’ve loved to have, or what the “you” of today would love to see the junior folks reading.